An A/D converter has been used to detect an electrical state of an internal circuit or a physical state of an electronic device for digital signal processing and convert an analog signal indicative of these states into a digital signal.
As a result of reviewing A/D conversions, the present inventors have recognized the following problems.
When an A/D converter is completely broken down and becomes inoperable, a digital signal does not correlate with an analog signal. In this case, it is possible to recognize an abnormality in a processor or a circuit at a subsequent stage that uses an output signal from the A/D converter.
However, incomplete deterioration or failure (hereinafter, referred to as “incomplete deterioration”) may occur in the A/D converter due to a secular change. The term “incomplete deterioration” refers to a failure mode in which an abnormality has occurred but the A/D converter operates at a glance and a certain output signal, although inaccurate, is generated. When an incomplete failure occurs, the subsequent processor or circuit operates according to an erroneous output signal, which causes malfunction of a system.